1. Field of the Invention
The present invention relates to a thermal image transferring device for thermally transferring toner images to both surfaces of a single recording medium and an image forming apparatus including the same.
2. Description of the Background Art
A printer, copier, facsimile apparatus or similar image forming apparatus of the type effecting the following image forming process is conventional. First, a photoconductive drum or similar image carrier is scanned imagewise to form a latent image thereon. Toner, charged to negative polarity or positive polarity, is deposited on the latent image to thereby produce a corresponding toner image. Subsequently, the toner image is transferred from the image carrier to a sheet or similar recording medium either directly or indirectly via an intermediate image transfer body and then fixed on the sheet by a thermal fixing device. An image forming apparatus of the type described must be provided with implementations that meet the increasing demand for high image forming speed.
For example, a switchback system and a one-pass system are known as systems capable of forming images on both sides of a single sheet. The switchback system forms an image on one surface of a sheet by conveying it via image transferring means and fixing means, turns the sheet, and then switches back the sheet toward the image transferring means and fixing means to thereby form an image on the other surface of the sheet. The one-pass system forms images on both surfaces of a sheet at the same time by conveying the sheet only one time.
More specifically, in a specific configuration of the one-pass system, a first toner image to be transferred to a first surface of a sheet is formed on a latent image carrier and then transferred to an intermediate image transfer body. Subsequently, a second toner image is formed on the latent image carrier. The second toner image and the first toner image carried on the intermediate image transfer body are simultaneously transferred to both surfaces of a single sheet conveyed to a nip between the latent image carrier and the intermediate image transfer body. The sheet is then conveyed to a thermal fixing device to have the toner images fixed thereon.
The one-pass system is free from various problems particular to the switchback system, e.g., high cost ascribable to a sophisticated switchback mechanism, long image forming time ascribable to switchback, and jam ascribable to the switchback of a sheet curled at the fixing means due to heat.
On the other hand, an electrostatic image transfer system and a thermal, simultaneous image transfer and fixation system are known in the art as systems for transferring a toner image from a photoconductive drum or similar image carrier or an intermediate image transfer body to a sheet. The electrostatic image transfer system effects image transfer by forming an electric field at a nip where the image carrier or the intermediate image transfer body, i.e., a donar and a sheet or acceptor contact each other. The thermal, simultaneous image transfer and fixation system heats a toner image carried on the donar to thereby soften it while causing the donar and a sheet to contact each other, and then separate the donar and sheet to thereby transfer the toner image to the sheet and fix the toner image. The thermal, simultaneous image transfer and fixation system is advantageous over the electrostatic image transfer system in that it obviates image degradation ascribable to toner scattering.
More specifically, the problem with the electrostatic image transfer system is that it is extremely difficult to cause the electric field to act only on the nip, i.e., the electric field extends to positions before and after the nip where the donar and sheet are spaced from each other. Toner or similar image forming agent, subject to the above electric field before and after the nip, flies from the donar and deposits on unexpected portions of the sheet. Such toner scattering causes black spots to appear around the resulting toner image or blurs the edges of the toner image.
Japanese Patent Laid-Open Publication No. 2000-250272, for example, discloses an image forming apparatus implementing both of the one-pass system and thermal, simultaneous image transfer and fixation system. This image forming apparatus includes a first and a second belt contacting each other while moving in the same direction (forward direction hereinafter) at a position where they contact each other.
More specifically, in the image forming apparatus taught in the above document, a first toner image formed on a photoconductive drum or image carrier is transferred to the first belt, which is moving in the forward direction in contact with the second belt. At the contact position, a heat roller for heating the first belt while supporting it and a press roller for heating the second belt while supporting it are positioned. The first toner image, electrostatically transferred from the drum to the first belt, is heated at the contact position to be thereby transferred to the second belt.
About the time when the above image transfer is effected, a second toner image is formed on the drum, electrostatically transferred to the first belt, conveyed to the contact position, and then brought into contact with one surface of a sheet. At this instant, the first toner image carried on the second belt is again conveyed to the contact position and brought into contact with the other surface of the sheet. The first and second toner images both are heated at the contact position to be thereby transferred to opposite surfaces of the sheet and fixed thereon.
Thus, the above image forming apparatus achieves the merits of both of the one-pass system and thermal, simultaneous image transfer and fixation system. Further, the apparatus does not directly heat the drum and therefore protects it from damage ascribable to temperature elevation while obviating image degradation.
However, the conventional image forming apparatus described above has the following problems left unsolved, Because the sheet, nipped between the first and second belts, must be heated from the inner surfaces of the belts, wasteful energy consumption ascribable to heat loss is critical. More specifically, when the fixation of a toner image on a sheet is effected independently of image transfer, it is a common practice to directly heat the sheet with a heat roller or similar heating means, efficiently transferring heat from the heating means to the sheet.
By contrast, in the thermal image transfer and fixation system that cannot directly heat a sheet, it is necessary to transfer the heat of the heat roller, pressure roller or similar heating means contacting the inner surface of the first or the second belt to the sheet indirectly via the belt. As a result, heat is stored in the first and second belts. Heat stored in the first and second belts is wastefully radiated because the first and second belts each move with both surfaces thereof being exposed to space. Moreover, the first belt must be intentionally cooled off by cooling means in order to obviate image degradation ascribable to the temperature elevation of the image carrier, as needed. These, in combination, noticeably increase wasteful energy consumption ascribable to energy loss.
The wasteful energy consumption stated above is more aggravated as a period of time over which the sheet and belt contact each other at the contact position is reduced. More specifically, when a sheet is indirectly heated via the belt, the outer surface of the belt is cooled due to heat transfer to the sheet despite that the inner surface is heated by the heating means. As a result, a temperature gradient occurs on opposite surfaces of the belt.
To heat a toner image to its melting point or softening point against the temperature gradient mentioned above, the heating temperature of the heating means must be made higher than the melting point or the softening point. For example, to heat a toner image at the contact position to 120° C., which is the softening point, the heating means must heat the belt to 140° C. higher than the softening point by 20° C. from the inner surface of the belt. At this instant, assume that the outer surface of part of the belt just preceding the contact position is 125° C., and that the outer surface of the belt and sheet contact each other for 0.5 second. Also, assume that it is desired to vary the contact time to 0.25 second, which is one-half of the above period of time, for heating the toner image to 120° C.
To implement the above temperature elevation, the same amount of heat as before the variation must be applied to the sheet and therefore toner image via the belt in one-half of the contact time, so that the temperature of the surface of the belt, starting contacting the sheet, must be raised. For example, it is necessary to raise the heating temperature of the heating means to 170° C. by 30° C. for thereby raising the temperature of the above belt surface to 135° C. higher than 125° C.
With the above scheme, it is possible to substantially double the amount of heat to be transferred to the sheet for a unit time, i.e., apply the same amount of heat as before the contact time is halved to the sheet. Despite that the contact time is halved, the temperature drop of the belt remains substantially the same because the amount of heat transferred from the belt to the sheet is the same. Consequently, the temperature of part of the belt moved away from the contact position is higher than before the variation. For example, when the contact time is 0.5 second or 0.25 second, the temperature of the above part of the belt is 120° C. or 130° C., respectively. In any case, the part of the belt moved away from the contact position must be cooled off to the desired level before reaching the image carrier, so that extra cooling is required as the contact time is reduced and aggravates heat loss.
Generally, in an image forming apparatus of the type fixing a toner image on a sheet with heat, heating means for fixation consumes more energy than the other structural parts. In this respect, the wasteful energy consumption ascribable to heat loss described above critically effects running cost and, in the worst case, increases the cost to an impractical degree. In this sense, it is preferable to confine the heating temperature of the heating means in a range higher than the melting point or the softening point of the image forming agent by 5° C. to 50° C.
Another problem with the image forming apparatus using both of the one-pass system and thermal, simultaneous image transfer and fixation system is that the leading edge positions of images formed on opposite surfaces of a sheet are shifted from each other for the following presumable reasons.
Usually, in the one-pass system, the second toner image formed on the first image carrier is transferred to a sheet at the nip between the first and second image carriers. On the other hand, the first toner image on the second image carrier may be transferred to the sheet at the above nip or at a different position on a sheet conveyance path. However, to implement image transfer at a position different from the nip, additional image transferring means for transferring the first toner image to the sheet is essential. Even when the first toner image is transferred at the nip, an arrangement must be made such that the leading edge of the first toner image enters the nip at the same time as the leading edge of the second toner image.
However, when toner is melted by heat as in the thermal image transfer and fixation system, the temperature of the first image carrier and that of the second image carrier rise due to heat applied during image transfer. As a result, the lengths of the endless paths along which the first and second image carriers move each increase in accordance with the temperature elevation and the coefficient of thermal expansion. It a difference in path length between the first and second image carriers varies, but a latent image representative of the second toner image is formed on the image carrier at fixed timing, then the timing at which each toner image enters the nip is shifted.
In the case of the electrostatic image transfer system that does not heat the image carrier during image transfer, the difference in path length between the first and second image carriers varies little. Therefore, only if a latent image representative of the second toner image is formed on the latent image carrier at fixed timing, the leading edges of the first and second toner images are shifted little from each other on the sheet.
Even when toner is melted by heat for transferring the first and second toner images to the sheet at the nip, the leading edge of each toner image is shifted little if the temperature of the image carrier raised during image formation is the same at all times. This is because the extension of the path length of the image carrier ascribable to thermal expansion remains the same during image formation, and therefore the difference in path length between the first and second image carriers does not vary during image formation. Therefore, if a latent image representative of the second toner image is formed at timing selected by taking account of the above extension, the leading edge of the toner image is shifted little as in the electrostatic image transfer system.
In practice, however, the temperature of each image carrier during image formation does not remain constant, depending on the condition in which the apparatus is operated. For example, each image carrier is operated over a longer period of time and more heated in a repeat print mode than in a single print mode. Consequently, the temperature of each image carrier and therefore the difference in path length varies from one mode operation to another mode operation. Particularly, when each image carrier is heated to 100° C. or above due to thermal image transfer, the shift of the leading edge positions is not negligible.